ABSTRACT: The ability to optically record neuronal electrical activity with the temporal and fine-feature waveform resolution on par with whole-cell patch clamp electrophysiology would permit the correlation, of the physiology of individual cells and cell types, to the neural circuit-level activity that underlies behaviors, cognition, and affective states observed in the normal and diseased brain. Genetically encoded voltage indicators (GEVIs) hold great promise for this purpose as cell-type specific probes, if their voltage-sensitivity, brightness, and temporal resolution can be engineered to provide the reliable detection of high frequency action potentials, the detection of sub-threshold ?minis? critical to synaptic scaling and homeostatic plasticity, and the ability to resolve waveforms useful for deducing specific channel/receptor contributions to spiking and synaptic transmission. We propose to invent next-generation GEVIs through rational design from first principles of non- biologically derived proteins. We will adapt artificial protein ?maquettes,? which are de novo-designed and rigid 4-helix bundle proteins that serve as custom scaffolds for arbitrarily positioning biological co-factors within the scaffold core. Strategic positioning of a biliverdin chromphore within a transmembrane maquette allows for voltage sensing by the optical Stark effect, in which chromophores exhibit electric field-induced changes in absorbance that result in ultrafast changes in observed fluorescence. We call these proteins, ?MASTERs? (Maquette Stark Effect Reporters). The ultrafast infrared-fluorescent reporters will recapitulate whole-cell recordings with no observable delay or waveform difference.